Angiogenesis, by definition, is the formation of new capillaries and blood vessels within living tissues; and is a complex process first recognized in studies of wound healing and then within investigations of experimental tumors. Angiogenesis is thus a dynamic process which involves extracellular matrix remodeling, endothelial cell migration and proliferation, and functional maturation of endothelial cells into mature blood vessels [Brier, G. and K. Alitalo, Trends Cell Biology 6: 454–456 (1996)]. Clearly, in normal living subjects, the process of angiogenesis is a normal host response to injury; and as such, is an integral part of the host body's homeostatic mechanisms.
It will be noted and appreciated, however, that whereas angiogenesis represents an important component part of tissue response to ischemia, or tissue wounding, or tumor-initiated neovascularization, relatively little new blood vessel formation or growth takes place in most living tissues and organs of mature adults (such as the myocardium of the living heart) [Folkman, J. and Y. Shing, J. Biol. Chem. 267: 10931–10934 (1992); Folkman, J., Nat. Med. 1: 27–31 (1995); Ware, J. A. and M. Simons, Nature Med. 3: 158–164 (1997)]. Moreover, although regulation of an angiogenic response in-vivo is a critical part of normal and pathological homeostasis, relatively little is presently known about the control mechanisms for this process.
Overall, a number of different proteins, growth factors and growth factor receptors have been found to be involved in the process of stimulation and maintenance of angiogenic responses. For example, a number of cell membrane-associated proteins are thought to be involved in the processes of angiogenesis. Such proteins include SPARC [Sage et al., J. Cell Biol. 109: 341–356 (1989); Motamed K. and E. H. Sage, Kidney Int. 51: 1383–1387 (1997)]; thrombospondin 1 and 2 respectively [Folkman, J., Nat. Med. 1: 27–31 (1995); Kyriakides et al., J. Cell Biol. 140: 419–430 (1998)]; and integrins αvβ5 and αvβ3 [Brooks et al., Science 264: 569–571 (1994); Friedlander et al., Science 270: 1500–1502 (1995)]. In addition, a major role is played by heparin-binding growth factors such as basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF); and thus the regulation of angiogenesis is believed today to involve matrix components such as extracellular heparin sulfate and core proteins such as syndecans which are found at the surface of endothelial cells.
However, while a number of heparin binding growth factors (including VEGF, FGF1 and FGF2) have been shown to promote angiogenesis in-vitro and in-vivo, their process involvement appears limited to tissues demonstrating some form of inflammatory response to trauma (as defined by the presence of blood-derived macrophages), be it a direct tissue injury (such as wounding) or ischemia. Moreover, the presence of blood-derived macrophages is also routinely associated with localized secretion of a number of proteins including cytokines such as IL-2 and TNF-α, growth factors such as VEGF and FGF-2, matrix metalloproteinases as well as many other biologically active molecules. Accordingly, although there have been many investigations, publications, and developments of these entities, there remains a general ignorance and failure of understanding by research investigators and clinicians alike regarding useful and effective specific means and methods for inducing angiogenesis on-demand within living cells, tissues, and organs. Thus, while the value and desirability of initiating new vascularization—especially using cells in localized areas on an as needed basis as well as a therapeutic treatment for individual patients—is well recognized, these aims remain a long sought goal yet to be achieved in a practical manner.